摘要： Colloidal semiconductor nanoplatelets (NPLs) have recently emerged as highly promising optical gain medium because of their superior optical properties. Here, the shell-thickness-dependent optical gain properties of CdSe/CdS core/shell NPLs synthesized by high-temperature growth are systematically investigated for the first time. The core/shell NPLs show the increased quantum yields and enhanced photostability as well as clear reduced emission blinking, thanks to the preferable passivation of nonradiative surface defects by the growth of the high-quality CdS shells under high reaction temperature. Meanwhile, the amplified spontaneous emission (ASE) performance of CdSe/CdS NPLs indicates a nonmonotonic dependence on the shell thickness. The ASE threshold is achieved as low as ≈4.4 μJ cm?2 for thick-shell NPLs with six monolayer CdS shells, and exhibiting ultrafast transient dynamics process (≈11 ps). Besides, an extremely long lifetime (>800 ps) and large bandwidth (>140 nm) of optical gain are observed by employing ultrafast transient absorption spectroscopy. Finally, a thick-shell NPLs vertical cavity surface-emitting laser is developed, which demonstrates spatially directional single-mode operation with an ultralow lasing threshold of ≈1.1 μJ cm?2. These excellent results are attributed to the remarkable optical gain performance of core/shell NPLs and represent an important step toward practical NPL laser devices.